Method for operating femtocell in wireless communication system

ABSTRACT

A method for operating a femtocell in a wireless communication system comprises the steps of: triggering the switching from the operation mode for providing a service to closed subscriber group (CSG) terminals which belong to a specific terminal group and to non-CSG terminals which do not belong to the specific terminal group, to the operation mode for providing the service only to said CSG terminals; and commanding said non-CSG terminals to change the operation mode, and carrying out a handover. The method of the present invention improves efficiency of femtocell operation, and reduces restrictions on terminals disposed in the vicinity of the femtocell. The method of the present invention can utilize the unused frequency range of the femtocell, thereby providing a service with a high data ratio.

TECHNICAL FIELD

The present invention relates to wireless communications and moreparticularly, to a method for operating a femtocell.

BACKGROUND ART

Along with the development of communications and the proliferation ofmultimedia technology, a variety of large-capacity transmissiontechniques have been adopted for wireless communication systems.Although wireless capacity can be increased by allocating more frequencyresources, there is a limit to allocating more frequency resources tomultiple users due to limited frequency resources. One approach toefficient utilization of the limited frequency resources is to scaledown the sizes of cells. In a smaller cell, a Base Station (BS) mayservice a reduced number of users and thus may allocate more frequencyresources to the users. A large-capacity service with better quality canbe provided to multiple users by reducing the sizes of cells.

Femtocells installed in houses and offices are a recent extensiveresearch area. A femtocell is an ultra-small mobile communication BSdeployed for indoor use such as a house or an office. While a femtocellis regarded as a similar type to a picocell, the former is more advancedthan the latter in terms of functions. The femtocell is connected to anInternet Protocol (IP) network available in the home or office andprovides a mobile communication service by accessing the Core Network(CN) of a mobile communication system through the IP network. Forexample, the femtocell is connected to the CN of the mobilecommunication system via a Digital Subscriber Line (DSL). In the mobilecommunication system, a user may receive a service from a legacymacrocell outdoors and from a femtocell indoors. Femtocells arecomplementary to legacy macrocells whose services get poor withinbuildings, thereby improving indoor coverage of the mobile communicationsystem. Since the femtocells serve only specified users, they canprovide high-quality voice and data services to the users. Furthermore,the femtocells can provide new services that are not available from thelegacy macrocells. The widespread use of femtocells is a driving forcebehind Fixed-Mobile Convergence (FMC) and reduces industrial cost.

A femtocell is personal communication equipment that an individualinstalls in his or her house or an office, accessible to specific usersonly. Once it is powered on, communication equipment installed in ahouse or an office is typically kept powered-on even though it is not inuse. To reduce interference between a femtocell and a macrocell, thefemtocell may operate in a different frequency band from that of amacrocell. When a User Equipment (UE) is located in the service area ofa femtocell inaccessible to the UE, the UE should use a frequency bandother than a frequency band allocated to the femtocell, irrespective ofwhether the frequency band of the femtocell is used or unused. Theresulting possible limit to the use of radio resources for UEs locatednear to the femtocell may lead to inefficient use of limited radioresources.

Accordingly, there exists a need for a method for efficiently operatinga femtocell.

DISCLOSURE

Technical Problem

An object of the present invention devised to solve the conventionalproblem is to provide a method for efficiently operating a femtocell.

Technical Solution

In an aspect of the present invention, a method for operating afemtocell in a wireless communication system includes triggeringswitching from an operation mode for providing a service to a ClosedSubscriber Group (CSG) terminal that belongs to a specific terminalgroup and to a non-CSG UE that does not belong to the specific UE groupto an operation mode for providing the service only to the CSG terminal,and performing handover for the non-CSG UE by commanding the non-CSGterminal to switch an operation mode of the non-CSG UE.

In another aspect of the present invention, a method for operating afemtocell in a wireless communication system includes triggeringswitching from an operation mode for providing a service only to aClosed Subscriber Group (CSG) terminal that belongs to a specificterminal group to an operation mode for providing the service to the CSGterminal and to a non-CSG UE that does not belong to the specificterminal group, and notifying the CSG terminal or the non-CSG terminalof the operation mode switching.

In a further aspect of the present invention, a method for performing ahandover operation in a wireless communication system having ahierarchical cell structure with a macrocell and a femtocell includesdetermining a handover criterion, and determining to perform handoveraccording to the handover criterion and performing the handover to amacrocell or a neighbor femtocell. The handover criterion is at leastone of a power-off state of a connected femtocell, whether or not a datarate of the connected femtocell is equal to or lower than a threshold,and a service policy of the femtocell.

Advantageous Effects

A femtocell can be efficiently operated and limitations imposed on a UEnear to the femtocell can be mitigated. In addition, the unusedfrequency band of the femtocell can be utilized, thereby enablingprovisioning of a high-rate service.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of a wireless communication system.

FIG. 2 illustrates an exemplary radio frame structure.

FIG. 3 illustrates an exemplary frame structure.

FIG. 4 illustrates exemplary mapping of physical resource units.

FIG. 5 illustrates a method for switching an operation mode of afemtocell according to an embodiment of the present invention.

FIG. 6 illustrates a method for switching an operation mode of afemtocell according to another embodiment of the present invention.

FIG. 7 illustrates exemplary deployment relationships between amacrocell and femtocells.

FIG. 8 illustrates a method for switching an operation mode of afemtocell according to a further embodiment of the present invention.

FIG. 9 is a block diagram of a User Equipment (UE).

FIG. 10 is a block diagram of a femtocell.

FIG. 11 is a block diagram of a wireless communication system includinga mobile femtocell according to an embodiment of the present invention.

FIG. 12 is a block diagram illustrating operation modes of a femtocellaccording to an embodiment of the present invention.

FIG. 13 is a flowchart illustrating a handover procedure according to anembodiment of the present invention.

BEST MODE FOR CARRYING OUT THE INVENTION

The technology as set forth herein is applicable to a variety ofwireless communication systems such as Code Division Multiple Access(CDMA), Frequency Division Multiple Access (FDMA), Time DivisionMultiple Access (TDMA), Orthogonal Frequency Division Multiple Access(OFDMA), Single Carrier Frequency Division Multiple Access (SC-FDMA),etc. CDMA can be implemented into a radio technology such as UniversalTerrestrial Radio Access (UTRA) or CDMA2000. TDMA can be implementedinto a radio technology such as Global System for Mobile communications(GSM)/General Packet Radio Service (GPRS)/Enhanced Data Rates for GSMEvolution (EDGE). OFDMA can be implemented into a radio technology suchas Institute of Electrical and Electronics Engineers (IEEE) 802.11(Wi-Fi), IEEE 802.16e (WiMAX), IEEE 802-20, Evolved UTRA (E-UTRA), etc.UTRA is a part of Universal Mobile Telecommunications System (UMTS).3^(rd) Generation Partnership Project (3GPP) Long Term Evolution (LTE)is a part of Evolved UMTS (E-UMTS) using E-UTRA. OFDMA is adopted fordownlink and SC-FDMA is adopted for uplink in 3GPP LTE. IEEE 802.16m isan evolution of IEEE 802.16e.

FIG. 1 is a block diagram of a wireless communication system. Thewireless communication system is deployed widely in order to providevarious communication services such as voice, packet data, etc.

Referring to FIG. 1, a typical wireless communication system includes aUser Equipment (UE) and a Base Station (BS). The UE is fixed or mobileand interchangeably used with a Mobile Station (MS), a User Terminal(UT), a Subscriber Station (SS), a wireless device, etc. The BS isgenerally a fixed station communicating with a UE. The term BS may bereplaced with a Node-B, a Base Transceiver System (BTS), an accesspoint, etc.

One BS may cover one or more cells.

BSs may be classified into femto BSs 20 and macro BSs 60 according totheir cell coverage or deployments. A cell covered by a femto BS 20 issmaller than a cell covered by a macro BS 60. The cell of the femto BS20 may wholly or partially overlap with the cell of the macro BS 60.This structure in which a wide cell is overlaid with a small cell isreferred to as a hierarchical cell structure.

The femto BS 20 may also be called a femtocell, a home Node-B, a homeeNode-B (HeNB), a Closed Subscriber Group (CSG) cell, etc. The macro BS60 may be called a macrocell, distinguishably from a femtocell.

The femto BS 20 is connected to a femto GateWay (GW) via an Iuhinterface. The Iuh interface interfaces between the femto BS 20 and thefemto GW 30 through an Internet Protocol (IP) network. The femto GW 30is an entity that manages at least one femto BS 20. To allow the femtoBS 20 to access a Core Network (CN) 90 of the wireless communicationsystem, the femto GW 30 may perform registration, authentication andsecurity procedures for the femto BS 20. The macro BS 60 is connected toa Radio Network Control (RNC) 70 via an Iub interface. The RNC 70 is anentity that manages at least one macro BS 60 and connects the macro BS60 to the CN 90. While the macro BS 60 is connected to the CN 90 via adedicated line, the femto BS 20 is connected to the CN 90 through the IPnetwork. While femto BSs 20 are shown herein as connected to the CN 90via the femto GW 30, they may be connected directly to the CN 90 withoutintervention of the femto GW 30.

UEs connected to the femto BSs 20 and UEs connected to the macro BS 60are referred to as femto UEs 10 and macro UEs 50, respectively. A femtoUE 10 may become a macro UE 50 through handover to a macro BS, and amacro UE 50 may become a femto UE 10 through handover to a femto BS.

The femto BSs 20 may be connected to the IP network wirelessly or bycable. For connectivity between the femto BSs 20 and the IP network, aknown module such as one conforming to wireless LAN, ZigBee, Power LineCommunication (PLC), Home Phone Networking Alliance (HomePNA), RS-485,etc. may be used. A femto BS connected wirelessly to the IP network isreferred to as a wireless femtocell. The wireless femtocell may use abattery for user convenience and thus a method for reducing batterypower consumption is needed for the wireless femtocell. To reduce powerconsumption of the femto BSs 20 and efficiently use limited radioresources, operation modes may be defined for the femto BSs 20. Forexample, the femto BSs 20 may provide services in the operation modes ofidle mode, active mode, etc. A wireless femtocell may be incorporatedinto a wireless Access Point (AP), with a wireless LAN function. Thewireless femtocell may be installed in a transportation vehicle. Thistype of femtocell which resides in a vehicle and thus has mobility isreferred to as a mobile femtocell.

Deployment Relationship between Macro BS and Femto GW>

The femto GW 30 may be configured in relation to the macro BS 60. Forinstance, the femto GW 30 and the macro BS 60 may be in a one-to-onecorrespondence. Or a femto GW 30 may be configured for each of aplurality of sectors covered by the macro BS 60. Or the cell area of themacro BS 60 is divided into a plurality of areas and a femto GW 30 isconfigured for each of the areas or each of groups into which the areasare grouped. Or a plurality of macro BSs 60 are grouped and a femto GW30 is configured for each macro BS group. Or a femto GW 30 may beconfigured for each tracking area. Or the management range of a femto GW30 may be defined irrespective of a macro BS 30 or a tracking area.

When a femto BS 20 or the macro BS 60 performs a self-organizationprocedure, the femto BS 20 or the macro BS 60 may request configurationinformation about the above-described femto GW 30 to the CN 90. Then theCN 90 may provide the configuration information about the femto GW 30 tothe femto BS 20 or the macro BS 60. When needed, the femto BS 20 or themacro BS 60 may transmit the configuration information about the femtoGW 30 to a femto UE 10 or a macro UE 50. The configuration informationabout the femto GW 30 may include IDentifier (ID) information of thefemto GW 30, etc.

In the wireless communication system, the macro BS 60 may use FractionalFrequency Reuse (FFR) to increase the efficiency of radio resources. Themacro BS 60 may determine an FFR factor, taking into account thedeployment of the femto BSs 20. The macro BS 60 may set a thresholdregarding the deployment of the femto BSs 20 and if the deployment ofthe femto BSs 20 is at or above the threshold, the macro BS 60 mayassign a low FFR factor to macro UEs 50. The threshold regarding thedeployment of the femto BSs 20 may be the number, density, etc. of thefemto BSs 20 deployed in a predetermined area.

The macro BS 60 may have a plurality of sectors and use differentfrequency bands in the different sectors. A femto BS 20 located in eachsector of the macro BS 60 may operate in the frequency band of anothersector. Hence, interference between the macro BS 60 and the femto BS 20can be mitigated. FFR may be implemented between femto BSs 20 via aradio interface between the femto BSs 20 or via the femto GW 30. Forexample, adjacent femto BSs 20 may allocate frequency bands byexchanging their FFR information between them, in a manner thatminimizes interference. Alternatively, a femto BS 20 may request afrequency band for FFR to the femto GW 30 and the femto GW 30 may thenallocate a frequency band for FFR to the femto BS 20, taking intoaccount the deployment of the femto BSs 20.

The macro BS 60 or the femto BSs 20 may broadcast information about thefemto BSs 20. The macro UEs 50 or the femto UEs 10 may acquireinformation about their neighbor femto BSs 20 by monitoring broadcastmessages from the femto BSs 20. The macro BS 60 or the femto BSs 20 maymulticast or unicast system information about the femto BSs 20, insteadof broadcasting the system information.

Cell ID sets of the macro BS 60 and the femto BSs 20 may be delivered inpredetermined different symbols or sequences. A femto UE 10 or macro UE50 in RRC_IDLE state or RRC_CONNECTED state may select or reselect acell by autonomous search or manual search. The automatic search is acell selection scheme in which the UE autonomously searches for a cellfor cell selection or reselection without receiving a command from a BSor being allocated a gap for the cell search from the BS. The manualsearch is a cell selection scheme in which a UE performs cell selectionor reselection using a gap allocated by a BS. A UE in RRC_CONNECTEDstate may notify the network that it needs a measurement gap forautonomous search, and the network may then allocate a measurement gapto the UE. With intra-frequency mobility, a UE in RRC_IDLE state mayselect a highest-ranking cell from among accessible femto BSs 20according to the best cell principle.

A UE may report the information indicating appropriate accessible femtoBSs to its serving BS. The serving BS is a femto BS or macro BS that isproviding a communication service to the UE. The UE may determine theappropriate accessible femto BSs based on its white list. The white listtabulates accessible femto BSs and contains information about the statesof the femto BSs, etc. The white list may be signaled by a higher layer.A femto BS 20 or the macro BS 60 may use a cell indicator (ex. 1 bit) ora specific Physical Cell ID (PCI) in order to indicate whether the BS isa femto BS. The cell indicator may be transmitted in a broadcast messageor a paging message.

Hereinbelow, a downlink (DL) refers to communication directed from a BSto a UE and an uplink (UL) refers to communication directed from a UE toa BS. A transmitter may be a part of the BS and a receiver may be a partof the UE on the downlink, whereas the transmitter may be a part of theUE and the receiver may be a part of the BS on the uplink.

The wireless communication system is not limited to any specificmultiple access scheme. Rather, a variety of multiple access schemes areavailable to the wireless communication system, such as CDMA, WCDMA,TDMA, FDMA, SC-FDMA, and OFDMA.

FIG. 2 illustrates an exemplary radio frame structure. The radio framestructure may be used for at least one of a macrocell and a femtocell ina hierarchical cell structure.

Referring to FIG. 2, a radio frame is divided into 10 subframes, eachsubframe including two slots. The slots of the radio frame are numbered0 to 19. Time taken to transmit one subframe is defined as aTransmission Time Interval (TTI). That is, the TTI is a scheduling unitfor data transmission. For example, one radio frame may be 10 ms, onesubframe may be lms, and one slot may be 0.5 ms, in duration.

A subframe may be divided into two slots in the time domain. A slot is aunit for allocating radio resources in the time and frequency domains.One slot may include a plurality of OFDM symbols in the time domain andat least one subcarrier in the frequency domain. For example, one slotmay include 7 or 6 OFDM symbols. A subframe may be divided into aplurality of Resource Blocks (RBs). An RB is a basic unit of radioresources allocated to a UE. An RB may include a plurality ofsubcarriers. For example, an RB may be defined by 12 contiguoussubcarriers in the frequency domain by two slots in the time domain. 10subframes may form one radio frame.

The frequency band of a subframe may be divided into three segments, ofwhich both side segments are allocated as a control region and themiddle segment is allocated as a data region. Since different frequencybands are allocated to the control region and the data region, thecontrol region and the data region are multiplexed in FDM (FrequencyDivision Multiplexing). However, this is purely exemplary and thus thelayout of the control region and the data region in a subframe is notlimited to the specific one. In addition, the number of subframes in aradio frame, the number of slots in a subframe, or the number of OFDMsymbols in a slot may vary.

Frequency hopping may occur between slots allocated to each UE in asubframe. That is, one frequency band at one side is allocated to the UEin one of the two slots and another frequency band at the other side isallocated to the UE in the other slot. As a control region resides inthe different frequency bands of the slots for the UE, a frequencydiversity gain can be achieved. Signals for a plurality of users may bemultiplexed in CDM (Code Division Multiplexing).

The above radio frame structure is purely exemplary and thus the numberof subframes in a radio frame or the number of slots in a subframe mayvary.

FIG. 3 illustrates an exemplary frame structure. In a hierarchical cellstructure, the frame structure may be used for at least one of amacrocell and a femtocell.

Referring to FIG. 3, a superframe includes a superframe header and fourframes FO, F1, F2 and F3. Each superframe and each frame are, by way ofexample, but not limited to, 20 ms and 5 ms respectively in duration.The superframe header may be located at the start of the superframe anda common control channel is allocated to the superframe header. Thecommon control channel delivers control information common to all UEswithin a cell, such as information about the frames of the superframe orsystem information. A synchronization channel may be disposed within thesuperframe header or in the vicinity of the superframe header totransmit a synchronization signal. The synchronization signal may carrycell information such as a cell ID.

One frame is divided into a plurality of subframes SF0, SF1, SF2, SF3,SF4, SF5, SF6 and SF7. Each subframe may be used for uplink or downlinktransmission. A subframe may include 6 or 7 OFDMA symbols, by way ofexample. A frame may be configured in Time Division Duplexing (TDD) orFrequency Division Duplexing (FDD). In TDD, each subframe is used foruplink transmission and downlink transmission at different time pointsin the same frequency. That is, the subframes of a TDD frame are dividedinto uplink subframes and downlink subframes in time. In FDD, eachsubframe is used for uplink transmission and downlink transmission indifferent frequencies at the same point of time. That is, the subframesof an FDD frame are divided into uplink subframes and downlink subframesin frequency. The uplink transmission and the downlink transmission maytake place simultaneously in different frequency bands.

A subframe is divided into one or more frequency partitions. Eachfrequency partition includes one or more Physical Resource Units (PRUs),that is, contiguous/localized and/or distributed/non-contiguous PRUs.Each frequency partition can be used for different purposes such as FFRor Multicast and Broadcast Services (MBS).

A PRU is a basic physical unit for resource allocation that includes aplurality of physically contiguous OFDMA symbols and a plurality ofphysically contiguous subcarriers. The number of OFDM symbols in the PRUmay be equal to the number of OFDMA symbols in a subframe. For instance,if one subframe includes 6 OFDMA symbols, the PRU may be defined as 18subcarriers by 6 OFDMA symbols. A logical Resource Unit (LRU) is a basiclogical unit for distributed and localized resource allocations. An LRUis defined as a plurality of OFDMA symbols and a plurality ofsubcarriers, including pilots used in the PRU. Accordingly, theeffective number of subcarriers in an LRU depends on the number ofallocated pilots.

A Distributed Resource Unit (DRU) may be used to achieve a frequencydiversity gain. The DRU contains a group of distributed subcarrierswithin a frequency partition. The physical size of the DRU is equal tothat of the PRU. One or more subcarriers may be a minimum unit ofphysically contiguous subcarriers that form each distributed subcarriergroup in a DRU.

A Contiguous Resource Unit (CRU) also known as a localized resource unitmay be used to achieve a frequency selective scheduling gain. The CRUcontains a localized subcarrier group. The physical size of the CRU isequal to that of the PRU. The CRU and the DRU may be supported in FDM inthe frequency domain.

FIG. 4 illustrates exemplary mapping of PRUs.

Referring to FIG. 4, the total subcarriers of a system bandwidth aredivided into PRUs. One PRU may include 18 subcarriers in frequency by 6or 7 OFDMA symbols in time. The number of OFDM symbols in the PRUdepends on a subframe type. Subframes may be categorized into Type 1with 6 OFDMA symbols and Type 2 with 7 OFDM symbols, to which thepresent invention is not limited. Thus, subframe types including variousnumbers of OFDM symbols such as 5 OFDMA symbols, 9 OFDMA symbols, etc.may be defined.

PRUs are divided into subbands and minibands according to apredetermined PRU partitioning rule (S110). A subband is a unit of PRUscontiguous in frequency or a minimum unit for forming CRUS. Thefrequency-domain size of the subband may be 4 PRUs. A miniband is a unitof distributed PRUs or a unit for forming DRUB and its frequency-domainsize may be 1 PRU or an integer multiple of 1 PRU. The total PRUs may begrouped by fours, that is, in units of four PRUs being the size of asubband and then allocated to subbands and minibands. A PRU used for asubband is denoted by PRU_(SB) and a PRU used for a miniband is denotedby PRU_(MB). The total number of PRUs is the sum of the number ofsubband PRUs PRU_(SB) and the number of miniband PRUs PRU_(MB). Thesubband PRUs PRU_(SB) and the miniband PRUs PRU_(MB) are reordered. Thesubband PRUs PRU_(SB) are numbered from 0 to (the number of subband PRUsPRU_(SB)−1) and the miniband PRUs PRU_(MB) are numbered from 0 to (thenumber of miniband PRUs PRU_(MB)−1).

The sequence of the miniband PRUs PRU_(MB) is permuted in the frequencydomain, that is, undergoes miniband permutation to ensure frequencydiversity in each frequency partition (S120). That is, the numberedminiband PRUs PRU_(MB) are permuted according to a predeterminedpermutation rule (or mapping rule). The permuted miniband PRUs aredenoted by PPRU_(MS) (permuted-PRU_(MS)).

Then the subband PRUs PRU_(sB) and miniband PRUs PRU_(MB) are allocatedto one or more frequency partitions. Each frequency partition issubjected to a cell-specific resource mapping procedure involvingCRU/DRU allocation, sector-specific permutation, subcarrier permutation,etc.

In this manner, PRUs are selected in fours and allocated to subbands orminibands. Therefore, the basic number of physically contiguous CRUs inthe frequency domain is 4. That is, a physically contiguous frequencyband in the frequency domain is configured in units of four CRUs. A partof four PRUs contiguous in the frequency domain may be allocated to userdata or a control signal using radio resources less than four PRUs.However, four or more PRUs contiguous in the frequency domain may not beallocated to user data or a control signal using radio resourcesrequiring more than four PRUs. In other words, a channel requiring fouror more contiguous PRUs is not supported. Thus there is a limit on radioresource allocation to data of various sizes (hereinbelow, data coversuser data and a control signal in its meaning). In addition, on userdata or a control signal that uses radio resources requiring four orfewer contiguous PRUs, there is also a constraint that the PRUs shouldbe subband PRUs.

In the wireless communication system, a system bandwidth may be dividedinto a plurality of frequency partitions which may be allocated for dataand/or a control signal of a macrocell, for a broadcast channel of themacrocell, and for a femtocell. The frequency partition for data and/ora control signal of a macrocell may be shared with a femtocell, but thefrequency partition for a broadcast channel of the macrocell may not beshared with the femtocell.

<Operation Modes of Femtocell>

Now a description will be given of a method for efficiently operating afemtocell. For efficient operations of the femtocell, its operationmodes are classified into (1) public mode, (2) private mode, and (3)flexible private mode.

In (1) public mode, the femtocell operates like a general BS. That is, apublic-mode femtocell may be accessible to all UEs, like a macrocell.The public-mode femtocell may be opened to a UE requesting access to thefemtocell and provide a service to the UE. The public-mode femtocell isreferred to as a public femtocell.

In (2) private mode, a femtocell provides a service only to particularUEs. UEs or a UE group that can access the private-mode femtocell iscalled a CSG (Closed Subscriber Group). The private-mode femtocell canprovide a service only to UEs of a CSG associated with the private-modefemtocell. The private-mode femtocell grants closed access to UEs. Theprivate-mode femtocell is referred to as a private femtocell.

In (3) flexible private mode, a femtocell provides a service to UEs of aCSG associated with the femtocell and to UEs that are not included inthe CSG, namely non-CSG UEs. It can be said that the flexible privatemode is a hybrid mode of the public mode and the private mode. Theflexible private-mode femtocell may provide a service differentially toa CSG UE and a non-CSG UE. The flexible private-mode femtocell assignsaccess priority to the CSG UE to thereby provide a service to the CSG UEwith priority over the non-CSG UE. The flexible private-mode femtocellis referred to as a flexible private femtocell.

The operation mode of a femtocell may be changed according to the policyof an owner or an operator or the service situation of the femtocell.For example, the private-mode femtocell may transition to the flexibleprivate mode according to the adjustment of an owner or an operator orthe service situation of the femtocell. The flexible private-modefemtocell may also transition to the private mode according to theadjustment of an owner or an operator or the service situation of thefemtocell.

Transition from Private Mode to Flexible Private Mode>

FIG. 5 illustrates a method for switching the operation mode of afemtocell according to an embodiment of the present invention.Specifically, FIG. 5 illustrates a procedure for switching a femtocellfrom the private mode to the flexible private mode and an accessprocedure of a non-CSG UE according to the operation mode switching ofthe femtocell.

Referring to FIG. 5, the operation mode of a femtocell may be switchedupon occurrence of an operation mode switching trigger (S210).Transition from the private mode to the flexible private mode may betriggered upon request of a CSG UE or non-CSG UE or according to theservice situation of the femtocell. A private femtocell may determine toswitch its operation mode, upon receipt of an access request from anon-CSG UE. Or the private femtocell may determine to transition to theflexile private mode, when a current data rate is low relative to itsavailable maximum data rate. For instance, when all CSG UEs within theprivate femtocell is in idle mode or sleep mode or when no CSG UE existswithin the private femtocell, the private femtocell may determine totransition to the flexible private mode. Or if a data rate provided to aCSG UE within the private femtocell is equal to or lower than apredetermined threshold, the private femtocell may determine totransition to the flexible private mode. If the remainder of thebandwidth of the private femtocell except for a bandwidth allocated toCSG UEs is equal to or higher than a threshold, the private femtocellmay determine to transition to the flexible private mode.

When determining to switch its operation mode, the femtocell transmitsan operation mode switch message to a femto GW or a CN of the wirelesscommunication system (S220). The operation mode switch message maycontain information about the cell ID, CSG ID, CSG list, operation mode,etc. of the femtocell. Operation mode switching may amount to new setupof the femtocell. Operation mode switching information may be includedin a setup message of the femtocell.

The femto GW or the CN may reply to the femtocell with a confirm messagein response to the operation mode switch message (S230). The confirmmessage may not be transmitted in response to the operation mode switchmessage according to system implementation.

The femtocell transitions to the flexible private mode (S240). Theflexible private femtocell is accessible to non-CSG UEs as well as CSGUEs. The flexible private femtocell may grant access and provide aservice to a CSG UE with priority over a non-CSG UE by assigning accesspriority to the CSG UE.

After transitioning to the flexible private mode, the femtocelltransmits an operation mode-indicating message (S250). The operationmode-indicating message may contain an indicator of one or more bitsindicating the operation mode of the femtocell. For example, if the bitvalue of the indicator is 0, this may indicate the private mode and ifthe bit value of the indicator is 1, this may indicate the flexibleprivate mode, or vice versa. That is, if the bit value of the indicatoris 1, this may indicate the private mode and if the bit value of theindicator is 0, this may indicate the flexible private mode. Theoperation mode-indicating message may be delivered in a ranging message,a paging message, a broadcast message, a synchronization signal, orsystem information. For instance, the indicator indicating the operationmode of the femtocell may be added to the ranging message, the pagingmessage, the broadcast message, the synchronization signal, or thesystem information to thereby indicate the operation mode of thefemtocell or the operation mode switching of the femtocell. Or theoperation mode switching of the femtocell may be indicated without usingan additional bit through phase shifting of the ranging message, thepaging message, the broadcast message, the synchronization signal, orthe system information. Or the Cyclic Redundancy Check (CRC) of theranging message, the paging message, the broadcast message, thesynchronization signal, or the system information may be masked by theindicator indicating the operation mode of the femtocell. The operationmode-indicating message may be transmitted in a higher layer messagesuch as a Radio Resource Control (RRC) message, a Media Access Control(MAC) message, or an L1/L2 message. The operation mode-indicatingmessage may be broadcast.

On the other hand, the private femtocell may indicate its operation modeto UEs by transmitting its CSG ID, whereas the flexible privatefemtocell may indicate its operation mode to UEs by not transmitting itsCSG ID. Or when the femtocell is in the private mode and the flexibleprivate mode, it may indicate its operation modes by means of differentcell IDs. That is, a cell ID for the private mode and a cell ID for theflexible private mode may be defined separately. Some of existingmacrocell IDs or newly defined cell IDs may be used for femtocells. Someof the femtocell IDs may be used for the private mode, while the otherfemtocell IDs may be used for the flexible private mode.

Upon receipt of the operation mode-indicating message, a non-CSG UE maytransmit a bandwidth request to the flexible private femtocell in orderto access the flexible private femtocell (S260). The non-CSG UE, whichis not allowed to access a private-mode femtocell, may be in idle modeor conduct communication by accessing a macrocell. The bandwidth requestmay be a message containing information needed for allocation of abandwidth or a bandwidth request indicator. The information needed forbandwidth allocation may contain a UE ID, a flow ID, a scheduling type,etc. The bandwidth request message may be a MAC message. Meanwhile, eachof a non-CSG UE and a CSG UE may have a CSG list or a whist list listingaccessible cells. Upon receipt of an operation mode-indicating messageindicating transition to the flexible private mode, both the non-CSG UEand the CSG UE may update information of their CSG lists or white lists.

The flexible private femtocell transmits an uplink (UL) grant message tothe non-CSG UE in response to the bandwidth request (S270). The UL grantmessage may include an acknowledgment for the bandwidth request, radioresource information for uplink transmission (the position and size ofradio resources), a UE ID, etc. The flexible private femtocell mayauthorize the non-CSG UE and grant access to the non-CSG UE throughauthentication.

The flexible private femtocell may transmit information about thenon-CSG UE requesting access to the femto GW or the CN (S280). The femtoGW or the CN may acquire the information about the non-CSG UE andperform procedures such as location update, paging, etc. for the non-CSGUE.

Upon receipt of the UL grant message, the non-CSG UE may transmit uplinkdata in the allocated uplink radio resources (S290).

<Transition from Flexible Private Mode to Private Mode>

FIG. 6 illustrates a method for switching the operation mode of afemtocell according to another embodiment of the present invention.Specifically, FIG. 6 illustrates a procedure for switching a femtocellfrom the flexible private mode to the private mode and an accessprocedure of a non-CSG UE according to the operation mode switching ofthe femtocell.

Referring to FIG. 6, the operation mode of a femtocell may be switchedupon occurrence of an operation mode switching trigger (S310).Transition from the flexible private mode to the private mode may betriggered upon request of a CSG UE or according to the service situationof the femtocell. A flexible private femtocell may determine to changeits operation mode, upon receipt of an access request from a CSG UE.Because the flexible private femtocell assigns access priority to a CSGGE, it may determine to transition to the private mode, upon request ofthe CSG UE for access. Or while the flexible private femtocell isproviding a service to a CSG UE and a non-CSG UE, if the data rate ofthe CSG UE is increased and thus the service to the non-CSG UE mayaffect the service to the CSG UE, the flexible private femtocell maydetermine to transition to the private mode. If a data rate provided toa CSG UE within the flexible private femtocell is equal to or higherthan a predetermined threshold, the flexible private femtocell maydetermine to transition to the private mode. If the remainder of thebandwidth of the flexible private femtocell except for a bandwidthallocated to CSG UEs is equal to or lower than a threshold, the flexibleprivate femtocell may determine to transition to the private mode.

When determining to switch its operation mode, the femtocell transmitsan operation mode switch message to the femto GW or the CN of thewireless communication system (S320). The operation mode switch messagemay contain information about the cell ID, CSG ID, CSG list, operationmode, etc. of the femtocell. Operation mode switching may amount to newsetup of the femtocell. Operation mode switching information may beincluded in a setup message of the femtocell.

The femto GW or the CN may reply to the femtocell with a confirm messagein response to the operation mode switch message (S330). The confirmmessage may not be transmitted in response to the operation mode switchmessage according to system implementation.

The flexible private femtocell transmits an operation mode switchmessage to UEs (S340). The operation mode switch message transmitted tothe UEs indicates that the flexible private femtocell will transition tothe private mode a predetermined time later. The operation mode switchmessage may be broadcast to all UEs, or may be multicast or unicast tonon-CSG UEs connected to the flexible private femtocell. The operationmode switch message may be transmitted a predetermined number of times,at every predetermined interval until before the operation mode of theflexible private femtocell is switched. The operation mode switchmessage may specify a time when the flexible private femtocell issupposed to transition to the private mode. The operation mode switchmessage may contain an indicator of one or more bits indicating theoperation mode switching of the femtocell. For example, if the bit valueof the indicator is 0, this may indicate operation mode non-switchingand if the bit value of the indicator is 1, this may indicate operationmode switching, or vice versa. That is, if the bit value of theindicator is 0, this may indicate operation mode switching and if thebit value of the indicator is 1, this may indicate operation modenon-switching. The operation mode switch message may be delivered in aranging message, a paging message, a broadcast message, asynchronization signal, or system information. For instance, theindicator indicating the operation mode switching of the femtocell maybe added to the ranging message, the paging message, the broadcastmessage, the synchronization signal, or the system information tothereby indicate the operation mode of the femtocell or the operationmode switching of the femtocell. Or the operation mode switching of thefemtocell may be indicated without using an additional bit through phaseshifting of the ranging message, the paging message, the broadcastmessage, the synchronization signal, or the system information. Or theCRC of the ranging message, the paging message, the broadcast message,the synchronization signal, or the system information may be masked bythe indicator indicating the operation mode of the femtocell, therebyindicating the operation mode switching of the femtocell. The operationmode switch message may be transmitted through a higher layer messagesuch as an RRC message, a MAC message, or an L1/L2 message.

Upon receipt of the operation mode switch message indicating transitionto the private mode, a non-CSG UE performs handover to a macrocell oranother neighbor femtocell (S350). That is, the operation mode switchmessage indicating transition to the private mode may be a handoverindication message for the connected non-CSG UE. Upon receipt of theoperation mode switch message indicating transition to the private mode,the non-CSG UE may update information of its CSG list or white list.

After transmitting the operation mode switch message to the UEs, theflexible private femtocell transitions to the private mode (S360). Thetransition to the private mode may occur a predetermined time aftertransmitting the operation mode switch message.

<Operation Mode Switching of Non-Overlapped Femtocell>

FIG. 7 illustrates deployment relationships between a macrocell andfemtocells.

Referring to FIG. 7, a femtocell may be deployed within the cell area ofa macrocell 160. This femtocell is referred to as an overlappedfemtocell 120. A femtocell may also be deployed outside the cell area ofthe macrocell 160. This cell is referred to as a non-overlappedfemtocell. The non-overlapped femtocell may be an independent femtocellthat does not belong to the cell area of any other BS of a differentsystem, or a semi-independent femtocell that belongs to the cell area ofa BS of a different system.

As described before, the overlapped femtocell 120 and the non-overlappedfemtocell 130 may switch their operation modes, when needed. When theoverlapped femtocell 120 transitions from the flexible private mode tothe private mode and thus commands handover to a connected non-CSG UE,the non-CSG UE can perform handover to a macrocell. On the contrary, ifthe non-overlapped femtocell 130 transitions from the flexible privatemode to the private mode and thus commands handover to a connectednon-CSG UE, the non-CSG UE cannot perform handover to a macrocell oranother femtocell. As a result, the non-CSG UE may not receive acommunication service. Accordingly, when the non-overlapped femtocell130 switches its operation mode, the communication service for thenon-CSG UE should be considered.

FIG. 8 illustrates a method for switching the operation mode of afemtocell according to a further embodiment of the present invention.Specifically, FIG. 8 illustrates a procedure for switching anon-overlapped femtocell from the flexible private mode to the privatemode and an access procedure of a non-CSG UE according to the operationmode switching of the non-overlapped femtocell.

Referring to FIG. 8, while the following description is given in thecontext of a non-overlapped femtocell, the same operation mode switchingmethod and the same access procedure for a non-CSG UE are alsoapplicable to an overlapped femtocell.

The operation mode of a femtocell may be switched, upon occurrence of anoperation mode switching trigger (S410). Transition from the flexibleprivate mode to the private mode may be triggered upon request of a CSGUE or according to the service situation of the femtocell. A flexibleprivate femtocell may determine to switch its operation mode, uponreceipt of an access request from a CSG UE. Because the flexible privatefemtocell assigns access priority to a CSG UE, it may determine totransition to the private mode, upon request of the CSG UE for access.Or while the flexible private femtocell is providing a service to a CSGUE and a non-CSG UE, if the data rate of the CSG UE is increased andthus the service to the non-CSG UE may affect the service to the CSG UE,the femtocell may determine to transition to the private mode. If a datarate provided to a CSG UE within the flexible private femtocell is equalto or higher than a predetermined threshold, the flexible privatefemtocell may determine to transition to the private mode. If theremainder of the bandwidth of the private femtocell except for abandwidth allocated to CSG UEs is equal to or lower than a threshold,the private femtocell may determine to transition to the private mode.

When determining to switch its operation mode, the femtocell transmitsan operation mode switch message to the femto GW or the CN of thewireless communication system (S420). The operation mode switch messagemay contain information about the cell ID, CSG ID, CSG list, operationmode, etc. of the femtocell. Operation mode switching may amount to newsetup of the femtocell. Operation mode switching information may beincluded in a setup message of the femtocell.

The femto GW or the CN may reply to the femtocell with a confirm messagein response to the operation mode switch message (S430). The femto GWmay notify the femtocell that the femtocell is a non-overlappedfemtocell, taking into account the deployment of the femtocell. Thefemto GW or the CN may transmit non-overlapped femtocell information inthe confirm message to the femtocell. Or the femtocell may already beaware that it is a non-overlapped femtocell. In this case, the femto GWor the CN may not transmit the non-overlapped femtocell information tothe femtocell. The confirm message may not be transmitted in response tothe operation mode switch message according to system implementation.

The femtocell transmits a service adjustment message to a non-CSG UE(S440). The service adjustment message may indicate a data rate decreaseto the non-CSG UE. The service adjustment message may be broadcast toall UEs, or may be multicast or unicast to non-CSG UEs connected to theflexible private femtocell. The service adjustment message may betransmitted a predetermined number of times, at every predeterminedinterval until before the data rate of the non-CSG UE is adjusted. Theservice adjustment message may specify a time when the data rate issupposed to be adjusted. The service adjustment message may contain anindicator of one or more bits indicating data rate adjustment. Forexample, if the bit value of the indicator is 0, this may indicate datarate non-adjustment and if the bit value of the indicator is 1, this mayindicate data rate adjustment, or vice versa. That is, if the bit valueof the indicator is 0, this may indicate data rate adjustment and if thebit value of the indicator is 1, this may indicate data ratenon-adjustment. The service adjustment message may be delivered in aranging message, a paging message, a broadcast message, asynchronization signal, or system information. For instance, theindicator indicating data rate adjustment may be added to the rangingmessage, the paging message, the broadcast message, the synchronizationsignal, or the system information to thereby indicate the data rateadjustment of the femtocell. Or the data rate adjustment of thefemtocell may be indicated without using an additional bit through phaseshifting of the ranging message, the paging message, the broadcastmessage, the synchronization signal, or the system information. Or theCRC of the ranging message, the paging message, the broadcast message,the synchronization signal, or the system information may be masked bythe indicator indicating the data rate adjustment of the femtocell, tothereby indicate that the data rate will be adjusted. The serviceadjustment message may be transmitted in a higher layer message such asan RRC message, a MAC message, or an L1/L2 message.

Upon receipt of the service adjustment message, a non-CSG UE transmitsan acknowledgement message to the femtocell (S450). The non-CSG UE firstverifies whether handover to a macrocell or another femtocell ispossible. If the non-CSG UE wants to receive a service at a low datarate from the serving femtocell, it transmits an acknowledgment messageto the femtocell. When determining to perform handover to a macrocell oranother neighbor femtocell, the non-CSG UE may perform handover to themacrocell or the neighbor femtocell without transmitting theacknowledgment message.

The femtocell provides the service at a lower data rate to the non-CSGUE (S460). Upon receipt of the acknowledgment message from the non-CSGUE, the femtocell is kept in the flexible private mode withouttransitioning to the private mode, while only reducing the data rate ofthe non-CSG UE. If the femtocell fails to receive the acknowledgmentmessage, the femto cell may transition to the private mode.

FIG. 9 is a block diagram of a UE.

Referring to FIG. 9, a UE 200 includes a processor 210, a memory 220, anRF unit 230, a display unit 240, and a user interface unit 250. Theprocessor 210 provides a control plane and a user plane by configuringradio interface protocol layers. The functionality of each layer may berealized through the processor 210. The processor 210 may perform anaccess procedure according to the operation mode of a femtocell, asdescribed before. The processor 210 may implement an operation of the UEaccording to a function of the femtocell, as described later.

The memory 220 is connected to the processor 210 and stores a UEoperating system, applications, and general files. The memory 220 mayinclude a USIM (Universal Subscriber Identity Module). Cell IDs, CSGIDs, a CSG list, a white list, etc. may be stored in the memory 220, forthe UE to access macrocells or femtocells.

The display unit 240 displays various types of information. The displayunit 240 may be configured with a well known component such as an LCD(Liquid Crystal Display), OLEDs (Organic Light Emitting Diodes), etc.The user interface unit 250 may be configured using user interfaces incombination, such as a keypad, a touch screen, etc. The RF unit 230 isconnected to the processor 210 and transmits and/or receives radiosignals.

FIG. 10 is a block diagram of a femtocell.

Referring to FIG. 10, a femtocell 300 includes a processor 310, a memory320, an RF unit 330, an IP interface unit 340, and a user interface unit350. The processor 310 provides a control plane and a user plane byconfiguring radio interface protocol layers. The functionality of eachlayer may be realized through the processor 310. The processor 310 maymanage the afore-described operation modes of the femtocell. Theprocessor 310 may distinguish a CSG UE from a non-CSG UE based oninformation stored in the memory 320. The processor 210 may perform alater-described function of the femtocell.

The memory 320 is connected to the processor 310 and stores a femtocelloperating system, applications, and general files. The memory 320 maystore CSG IDs, a CSG list, a white list, etc. in order to distinguishCSG UEs from non-CSG UEs.

The IP interface unit 340 supports connectivity to an IP network and mayuse a module such as a known module conforming to wireless LAN, ZigBee,PLC, HomePNA, or RS-485.

The user interface unit 350 may be configured with known user interfacesin combination, such as a keypad, a touch screen, etc. A user mayadjust/switch the operation mode of the femtocell through the userinterface unit 350. The RF unit 230 is connected to the processor 310and transmits and/or receives radio signals.

<Movement of Femtocell>

FIG. 11 is a block diagram of a wireless communication system includinga mobile femtocell according to an embodiment of the present invention.

Referring to FIG. 11, a mobile femtocell 230 refers to a femtocell thatis installed in a transportation vehicle like an aircraft, a ship, atrain, a car, etc. and thus has mobility. On the other hand, a femtocellthat is installed in a home or an office and thus does not have mobilityis a fixed femtocell 220.

The mobile femtocell 230 may configure a radio interface X2 with aneighbor macrocell 260 or the fixed femtocell 220, as the transportationvehicle carrying the mobile femtocell 230 moves. The mobile femtocell230 may configure an interface S1 with a CN 290 of the wirelesscommunication system. The mobile femtocell 230 may be connected to theCN 290 through an IP network (i.e. a backbone network). The macrocell260 and the fixed femtocell 220 also configure interfaces S1 with the CN290.

(1) Exchange of Information between Mobile Femtocell and FixedFemtocell/Macrocell

The mobile femtocell 230 may transmit information about UEs connected toit, information about the white list of each UE, etc. to the neighborfixed femtocell 220 or the neighbor macrocell 260 via the radiointerface X2 or the backbone network S1. For instance, when the mobilefemtocell 230 moves toward the fixed femtocell 220 or the macrocell 260,the mobile femtocell 230 requests configuration of the radio interfaceX2 to the fixed femtocell 220 or the macrocell 260. Upon configurationof the radio interface X2, the mobile femtocell 230 may transmitinformation about UEs connected to it, information about the white listof each UE, etc. to the fixed femtocell 220 or the macrocell 260 via theradio interface X2. Or the mobile femtocell 230 may notify the fixedfemtocell 220 or the macrocell 260 of its approach via the backbonenetwork S1 configured with the CN 290 and may transmit the informationabout UEs connected to it, the information about the white list of eachUE, etc. to the fixed femtocell 220 or the macrocell 260 via thebackbone network S1.

The fixed femtocell 220 or the macrocell 260 may request the informationabout UEs connected to the mobile femtocell 230, the information aboutthe white list of each UE, etc. to the mobile femtocell 230 via theradio interface X2 or the backbone network S1. Then the mobile femtocell230 may transmit the information about UEs connected to the mobilefemtocell 230, the information about the white list of each UE, etc. tothe fixed femtocell 220 or the macrocell 260.

Meanwhile, the mobile femtocell 230 may broadcast its system informationto the neighbor fixed femtocell 220, the neighbor macrocell 260, oradjacent UEs (not shown). Or the mobile femtocell 230 may multicast orunicast its system information. The mobile femtocell 230 may transmit acell identification indicator in the system information. The cellidentification indicator is an indicator (e.g. a 1-bit indicator)identifying the mobile femtocell 230 from the fixed femtocell 220 and/orthe macrocell 260. Or a specific cell ID may be used to identify themobile femtocell 230. The specific cell ID may be an existing PCID or anewly defined PCI. The mobile femtocell 230 may operate in apredetermined frequency band and thus it may be identified based on thepredetermined frequency band.

The mobile femtocell 230 may preserve information about a departure anda destination. When arriving at the destination with information aboutUEs connected to the mobile femtocell 230 preserved, the mobilefemtocell 230 may transmit the information about the connected UEs tothe fixed femtocell 220 or the macrocell 260 at the destination. Theinformation about the connected UEs may be transmitted via the radiointerface X2 configured with the fixed femtocell 220 or the macrocell260 or via the backbone network S1. In the case where the mobilefemtocell 230 has information about the departure and the destination,it may not transmit the information about the connected UEs to the fixedfemtocell 220 or the macrocell 260 that are located in a path.Meanwhile, the mobile femtocell 230 may provide a white list associatedwith the fixed femtocell 220 or the macrocell 260 at the destination toa UE carried in the transportation vehicle.

(2) Connectivity of Mobile Femtocell

Information about UEs of passengers may be provided to the mobilefemtocell 230 installed in the transportation vehicle and thus themobile femtocell 230 may restrict access according to the information.That is, the mobile femtocell 230 may generate a CSG UE list byacquiring information about the UEs within the transportation vehicle.The mobile femtocell 230 may provide its information as a white list tothe UEs of the passengers. The UEs may write the mobile femtocell 230 intheir white lists and access the mobile femtocell 230 using the whitelists.

The mobile femtocell 230 may configure an inter-BS interface X2 with aneighbor macrocell, microcell, mobile/fixed femtocell, picocell, etc.The mobile femtocell 230 may be a public femtocell accessible to allUEs, a private femtocell accessible only to particular UEs, or aflexible private femtocell.

Meanwhile, the mobile femtocell 230 may charge users connected to it forservices by transmitting information about the users to the network. AUE may request an authentication token for accessing the mobilefemtocell 230 to the mobile femtocell 230 or to the neighbor fixedfemtocell 220 or macrocell 260. The cell determines whether the UE canaccess the mobile femtocell 230 by checking whether the UE has beencharged for services, authenticated, etc. The cell may assign anauthentication token for accessing the mobile femtocell 230 to the UEthat can access the mobile femtocell 230. The UE may perform an accessprocedure to the mobile femtocell 230 using the authentication token.The authentication token that grants the UE with access to the mobilefemtocell 230 may be maintained or extinguished according to a conditionsuch as a predetermined time, period, area, etc.

The above-described information about connectivity to the mobilefemtocell 230 may be broadcast, multicast, or unicast to the UE.

(3) Channel of Mobile Femtocell

The mobile femtocell 230 may share all or a part of a channel (afrequency band) used by the macrocell 260. Alternatively, the mobilefemtocell 230 may use a specific frequency band different from thefrequency band of the macrocell 260. The mobile femtocell 230 may shareall or a part of the frequency band of a channel used by the fixedfemtocell 220 or may operate in a different frequency band from thefrequency band of the fixed femtocell 220. Information about a channel(a frequency band) used by the mobile femtocell 230 may be broadcast,multicast, or unicast. The mobile femtocell 230 may configure aninter-BS interface X2 with a neighbor mobile/fixed femtocell andexchange channel information with the neighbor mobile/fixed femtocellvia the interface X2 or the backbone network.

A UE may identify the mobile femtocell 230 based on an indicator orspecific PCID indicating the mobile femtocell 230 and may thus be awareof the frequency band of the mobile femtocell 230.

Synchronization is acquired between the mobile femtocell 230 and themacrocell 260 in a GPS-based, Precision Timing Protocol (IEEE 1588,PTP)-based, or radio interface-based manner. Or the mobile femtocell 230may acquire synchronization to the macrocell 260 by acquiringinformation about the macrocell 260 from an adjacent UE. The macrocell260 may transmit a predetermined pilot pattern at every predeterminedinterval and the mobile femtocell 230 may acquire synchronization to themacrocell 260 by receiving the predetermined pilot pattern.

<Operation Modes of Femtocell>

FIG. 12 is a block diagram illustrating operation modes of a femtocellaccording to an embodiment of the present invention.

Referring to FIG. 12, even though a number of femtocells are deployedwithin a macrocell, all femtocells do not provide services to UEs.Nonetheless, the femtocells transmit system information periodically.For transmission of system information, a broadcast channel should beallocated to each femtocell. As a result, problems such as radioresource waste and unnecessary cell measurement of an adjacent UE mayoccur. In order to efficiently use limited radio resources and reduceinter-cell interference and the power consumption of femtocells,operation modes may be defined and operated for the femtocells.

A femtocell may be placed in a power-on or power-off state. In thepower-on state, the femtocell may operate in active mode, idle/sleepmode, power adjust mode, etc. The power-off state of the femtocellincludes an expected power-off state and a sudden power-off state.

The active mode refers to a state in which the femtocell is providing acommunication service to a UE, whereas the idle/sleep mode is a state inwhich the femtocell is not providing a communication service to any UE.The power adjust mode refers to a state in which the femtocell adjustsDL/UL power. For instance, the femtocell may adjust DL/UL poweraccording to its operation in the active mode or idle/sleep mode or tomitigate inter-cell interference according to self-organization oroperation mode switching of a neighbor femtocell.

The femtocell is placed in the expected power-off state, when it expectspower-off, while it is in the sudden power-off state when power is offunexpectedly. For example, when the femtocell is disconnected from an IPnetwork or the transmission rate of the IP network drops to or below apredetermined minimum transmission rate, the femtocell may transition tothe expected power-off state. On the other hand, if power is blocks allof sudden or a radio interface is blocked between the femtocell and aUE, the femtocell may transition to the sudden power-off state.

A. Transition from Active Mode to Idle/Sleep mode

An idle-mode/sleep-mode femtocell transmits system information in alonger period than an active-mode femtocell. An idle-mode femtocell maytransmit system information in a longer period than a sleep-modefemtocell. When all UEs to be serviced move out of the cell area of thefemtocell or perform handover to other femtocells or macro cells, thefemtocell may transition to the idle mode or sleep mode. Or when all UEsto be serviced transition to idle mode or sleep mode, the femtocell mayalso transition to the idle mode or sleep mode. Herein, the femtocellmay switch its operation mode to the idle/sleep mode a predeterminedmode switching time later. The mode switching time may be a time neededfor the femtocell to switch its operation mode or a time by which theoperation mode switching is delayed. When determining to switch itsoperation mode, the femtocell may notify the femto GW or the CN of itsoperation mode switching. The operation mode switching may be signaledto the femto GW or the CN in a setup message of the femtocell.

B. Transition from Idle/Sleep Mode to Active mode

A femtocell may transition to the active mode upon request of a UE. TheUE may request the femtocell to transition to the active mode, based onits white list. The UE may receive the white list from a higher layer.The idle-/sleep-mode femtocell may wake up at every predeterminedinterval and monitor whether there is a UE to be serviced around thefemtocell. The UE may request the femtocell to transition to the activemode in a listening interval during which the femtocell is awake. Or theUE may request the femtocell's transition to the active mode to amacrocell covering the area of the femtocell and then the macrocell mayrequest the femtocell to transition to the active mode.

Meanwhile, the femto GW or the CN may switch the operation mode of afemtocell. An active-mode femtocell notifies the femto GW or the CN thatit is not servicing any UE and the femto GW or the CN may command thefemtocell to transition to the idle/sleep mode. Specifically, the femtoGW or the CN may determine the service state of a neighbor femtocell andmay command switching of the operation mode of the femtocell accordingto the situation of the neighbor femtocell. For example, if a largenumber of UEs are connected to the neighbor femtocell and thus a UEneeds handover to the femtocell, the femto GW or the CN may command thefemtocell to operate in the active mode. The femto GW or the CN maycommand switching of the operation mode of the femtocell, taking intoaccount the neighbor femtocell and mobility of UEs around the femtocell.For instance, the femto GW or the CN determines whether a UE isapproaching the femtocell and may command the femtocell to transitionfrom the idle/sleep mode to the active mode according to thedetermination. The femto GW or the CN may command the femtocell toswitch its operation mode, upon request of the neighbor femtocell. Forinstance, when a neighbor femtocell requests a specific femtocell totransition to the idle/sleep mode, the femto GW or the CN may determinewhether the specific femtocell can transition to the idle/sleep mode andmay command the specific femtocell to transition to the idle/sleep modeaccording to the determination.

C. Transition from Power-On State to Sudden Power-Off state

A femtocell is connected to a mobile communication CN via an IP networkin a home or an office. The IP network is a public network that multipleusers access. During a communication service for a UE, the UE may besuddenly disconnected from the IP network or a network transmission ratemay be decreased to or below a threshold. In addition, the user may turnoff the femtocell or the femtocell may be turned off due to a blackoutduring the communication service in progress. The sudden power-off ofthe femtocell may cause interruptions to voice service and thus maydegrade the quality of data service. When the UE is disconnected fromthe femtocell, it should perform fast handover to a neighbor femtocellor a macrocell to receive the on-going communication service seamlessly.

There exists a need for a method for enabling the UE to rapidly identifythe sudden power-off of the femtocell. For this purpose, the femtocellmay periodically transmit information indicating its power-on in theactive mode and/or the idle/sleep mode. If a UE has not received theinformation indicating power-on from the femtocell, it may determinesudden power-off of the femtocell. Then the UE performs handover to aneighbor femtocell or a macrocell. Meanwhile, the UE may determine thesudden power-off of the femtocell, if it monitors system information, abroadcast signal, etc. from the femtocell and has not received a signalfor a predetermined time from the femtocell. Upon detection of thesudden power-off of the femtocell, the UE may update state informationof the femtocell in its white list. During the UE's handover to theneighbor femtocell or the macrocell, the femtocell may transmit itsstate information to a target BS.

When an idle-mode or sleep-mode UE attempts to communicate with afemtocell within the femtocell, it should be able to detect thepower-off state of the femtocell. If the UE has not received a signalfrom the femtocell for a predetermined time or the UE receives a signalat a data rate equal to or lower than a threshold from the femtocell,while monitoring system information, a broadcast signal, etc. from thefemtocell, the UE may notify a neighbor femtocell or a macrocell of thesudden power-off of the femtocell. The data rate threshold of thefemtocell may be predetermined or differentiated depending oncommunication service types. For example, the data rate threshold may beset to be high for a real-time communication service and to be low for anon-real-time communication service.

The macrocell may store a list of UEs connected to a femtocell withinits coverage and may grant access to a UE that attempts handover due tothe sudden power-off of the femtocell, with priority. For instance, a UEthat attempts handover due to the sudden power-off of a femtocell mayperform handover in a non-contention-based manner or using an IDpre-allocated by the macrocell.

D. Transition to Power Adjust Mode

Transmission power control of a femtocell is most significant inreducing interference between the femtocell and a macrocell or aneighbor femtocell. While research is underway as to how to determinethe maximum and minimum transmission power of a femtocell, there is nospecified method for determining the transmission power of a femtocell,taking into account the operation mode of the femtocell.

The femtocell may control the transmission power of system information,a broadcast signal, etc. as well as their transmission intervalsaccording to the operation mode of the femtocell. For example, anactive-mode femtocell may provide a communication service with hightransmission power, whereas an idle-mode/sleep-mode femtocell mayprovide a communication service with low transmission power. Control ofthe transmission power of a femtocell according to its operation modemeans that the cell coverage of the femtocell is changed according toits operation mode. If the cell coverage of the femtocell is changed, aneighbor femtocell list may be changed and radio resources may beallocated to neighbor femtocells in a different manner. That is, thechange of the cell coverage of a femtocell may change the interferencestructure between femtocells and to control the interference structure,the power of femtocells may be controlled.

A femtocell may controls its transmission power to a predeterminedlevel, based on interference that the femtocell measures with respect toa neighbor femtocell, based on an interference measurement report from aUE, or according to a command from the network, depending on whether thefemtocell is in the active mode or the idle/sleep mode. When determiningto control its transmission power, the femtocell may broadcast powercontrol information in system information, or may multicast or unicastthe power control information to specific UEs.

A femtocell operates in power adjust mode, taking into account itspower-on/off that may occur at any time. For example, if a secondfemtocell is installed through self-organization near to a firstfemtocell that has appropriate cell coverage, the first and secondfemtocells need to adjust their cell coverage in order to reduceinterference between them. To control the cell coverage between thefemtocells, (a) the femtocells may exchange transmission powerinformation by establishing an interface between them. Or (b) thefemtocells may provide their own transmission power information over thenetwork and receive a transmission power control message from thenetwork. Or (c) the femtocells may control their cell coverage accordingto an interference report from a UE.

E. Transition from Power-On State to Expected Power-Off State

To transition to the expected power-off state, a femtocell may broadcastinformation about the start time of the power-off state, the duration ofthe expected power-off state, etc. to UEs. The femtocell may transmitthe information about the start time of the power-off state, theduration of the expected power-off state, etc. to the femto GW or theCN. The CN may transmit the information to a neighbor femtocell or amacrocell. A UE may write the expected power-off state in its whitelist. The UE may notify the neighbor femtocell or the macrocell of theexpected power-off state of the femtocell. The neighbor femtocell or themacrocell may be aware of the expected power-off state of the femtocellby receiving a report from the UE or related information from the CN andmay write the expected power-off state of the femtocell in its neighborBS list.

<Handover from Femtocell to Macrocell or Femtocell>

FIG. 13 is a flowchart illustrating a handover procedure according to anembodiment of the present invention.

Referring to FIG. 13, a UE connected to a femtocell determines a definedhandover criterion (S510). The UE determines whether to perform handoverto a macrocell or a neighbor femtocell by determining the handovercriterion. Handover from a femtocell to a macrocell or a neighborfemtocell is called outbound handover.

Outbound handover criteria may be defined as follows.

(1) Signal strength measurement: when the UE moves out of the cellcoverage of the femtocell, the UE measures the strengths of signalsreceived from neighbor cells and performs handover to a cell having thegreatest signal strength.

(2) Emergency call: the UE is connected to the femtocell according to anemergency call. Upon expiration of an emergency call condition, the UEperforms handover to a macrocell.

(3) Power-off state of femtocell: when the femtocell is placed in thepower-off state, the UE performs handover to a macrocell or a neighborfemtocell.

(4) Data rate threshold of femtocell: if the data rate of the femtocelldrops to or below a predetermined threshold, the UE performs handover toa macrocell or a neighbor femtocell. The threshold may be an absolutelowest data rate determined irrespective of communication service typesor may be a relative service-based data rate set according to a servicetype (e.g. VoIP, a data packet, MBS, etc.).

(5) Service policy of femtocell: when a use condition of the UE isreleased according to a service policy of the femtocell, the UE performshandover to a macrocell or a neighbor femtocell. The femtocell mayrequest handover to the UE according to its service policy. The servicepolicy may include the type, use time, etc. of a service assigned to theUE in case of a charging femtocell, and a movement path, area, etc. incase of a mobile femtocell.

The UE performs handover to the macrocell or the neighbor femtocellaccording to the result of the determination made as to the handovercriterion (S520). The UE may set at least one of the above-describedhandover criteria as its handover criterion. If the handover criterionis met, the UE performs an outbound handover procedure.

All of the above-described functions may be implemented by a processorsuch as a microprocessor, a controller, a microcontroller, or anApplication Specific Integrated Circuit (ASIC) in software or programcode created to perform the functions. The design, development, andexecution of the code will be apparent to those skilled in the art basedon the description of the present invention.

While the present invention has been described referring to theembodiments set forth above, those skilled in the art will appreciatethat many variations and modifications may be made without departingfrom the spirit and scope of the present invention. The scope of theinvention should be determined by the appended claims and their legalequivalents, not by the above description.

1. A method for operating a femtocell in a wireless communicationsystem, the method comprising: triggering switching from an operationmode for providing a service to a Closed Subscriber Group (CSG) terminalthat belongs to a specific terminal group and to a non-CSG UE that doesnot belong to the specific UE group to an operation mode for providingthe service only to the CSG terminal; and performing handover for thenon-CSG UE by commanding the non-CSG terminal to switch an operationmode of the non-CSG UE.
 2. The method according to claim 1, wherein theCSG terminal has access priority over the non-CSG terminal.
 3. Themethod according to claim 1, further comprising notifying a femtogateway or a core network of the operation mode switching.
 4. A methodfor operating a femtocell in a wireless communication system, the methodcomprising: triggering switching from an operation mode for providing aservice only to a Closed Subscriber Group (CSG) terminal that belongs toa specific terminal group to an operation mode for providing the serviceto the CSG terminal and to a non-CSG UE that does not belong to thespecific terminal group; and notifying the CSG terminal or the non-CSGterminal of the operation mode switching.
 5. The method according toclaim 4, further comprising, upon receipt of an access request from thenon-CSG terminal, transmitting information about the non-CSG terminal toa femto gateway or a core network.
 6. A method for performing a handoveroperation in a wireless communication system having a hierarchical cellstructure with a macrocell and a femtocell, the method comprising:determining a handover criterion; and determining to perform handoveraccording to the handover criterion and performing the handover to amacrocell or a neighbor femtocell, wherein the handover criterion is atleast one of a power-off state of a connected femtocell, whether or nota data rate of the connected femtocell is equal to or lower than athreshold, and a service policy of the femtocell.